I encountered a minor simulation issue while using the Multisim SPICE tool to analyze an oscillator.
This is a simple Colpitts oscillator circuit, and the simulation shows that it indeed oscillates. However, I wanted to view the transfer function of the discrete components just to ensure everything is working properly. When I tried to get the Bode Plotter to work, I started running into some trouble (Figure 1).
Figure 1: Unsuccessful analysis attempt.
Just using the Bode Plotter on the discrete component network showed me what I originally hoped to see: a peak in the transfer function at the correct frequency—in this case, about 1.6 MHz. The signal source V1 provides the circuit excitation needed for the Bode Plotter to function properly.
However, when I tried to use the Bode Plotter on the oscillator circuit, even though the oscillator was functioning properly and transmitting signals, there was no response. The Bode Plotter does not accept the oscillation of the oscillator itself as circuit excitation. I had to deal with more parts (Figure 2).
Figure 2: Successful analysis attempt.
As shown in Figure 2, I had to add V2 and R4 as external stimuli to persuade the Bode Plotter to do its job. (Isn’t it funny how you can anthropomorphize an inanimate object?)
It certainly didn’t take long. I used a 1 mV peak for V2 and fed it to the top of R3 through a 1 MΩ resistor. It can be safely said that V2 and R4 have a negligible destructive effect on the oscillator circuit, but since these two parts can be included in the simulation, the Bode Plotter seemed to come back to life and gave me a usable result.
Perhaps I could have connected a 1 mV signal to R3 or something similar, but since I got the result I needed, I didn’t pursue it further.
It is certain that the Bode plot of the discrete components and the Bode plot of these components in the oscillator differ slightly, but I choose to attribute this difference to minor loading effects, possibly influenced by the gate of the 2N4392.
But the result was good enough!
Author: John Dunn
Translated by: Susan Hong
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